Practical self-propelled wheelchairs were first made possible in the 1950's and 1960's, as practical and relatively inexpensive rechargeable battery technology became available. Since their introduction, numerous designs have been proposed to improve traction and maneuverability of the two-wheel drive self-propelled wheelchair, many of which involve elaborate "endless belt" drive systems, as for example shown in U.S. Pat. No. 2,765,860 to Church, issued Oct. 9, 1956; U.S. Pat. No. 2,751,027 to McLaughlin, issued June 19, 1956; U.S. Pat. No. 4,077,483 to Randolph, issued Mar. 7, 1978; and U.S. Pat. No. 4,119,163 to Ball, issued Oct. 10, 1978. While each of the wheelchairs disclosed in those patents are relatively mobile and capable of negotiating obstacles, they are all characterized by elaborate designs implementing many components to carry the endless belts and provide the driving force thereto, and thus are relatively heavy or have a high degree of rolling friction. Thus, inordinate quantities of electrical energy are required to drive these chairs resulting in relatively limited range between battery recharge. In addition, they are difficult to transport, require relatively high levels of maintenance, and can be destructive of certain flooring materials, such as carpet, due to differential steering.
Thus, four-wheel drive endless belt wheelchairs not been well accepted. For similar reasons, wheeled-only four-wheel drive wheelchairs have also met with little success, as they too necessitate differential steering, for example as employed in a "bobcat" front-end loader, which like in the case of endless belt systems can impose an inordinate drain on the wheelchair batteries, and can be destructive of floor coverings. However, the maneuverability afforded by either endless belt or four-wheel drive does have advantages worth pursuing. For instance, four-wheel drive greatly enhances the climbing ability of the wheelchair, for example, to transverse small or low obstacles, and improves traction significantly on slippery or disintegrated surfaces, and relatedly improves braking ability, especially when proceeding down an incline.